1. Field of the Invention
The present invention relates to an optical scanning device for optically scanning a scan object surface and an image forming apparatus including the same such as a copier, a printer, a facsimile machine, and a plotter.
The present invention can be applied to an optical scanning type barcode reader, an on-vehicle laser radar device, etc.
2. Description of the Related Art
In conventional optical scanning devices, a polygon mirror or a galvanometer mirror is employed as a deflector for scanning light beams. In order to achieve images of higher resolution and higher printing speeds, the deflector needs to be rotated at higher speed. However, there are limitations in high-speed scanning due to problems such as durability of the bearings and heat and noise caused by windage.
To address these problems, in recent years and continuing, research for deflectors employing silicon micro-machining is in progress. For example, Patent Documents 1, 2 disclose an oscillating mirror and a torsion beam pivotally supporting the oscillating mirror integrally formed with a Si substrate.
According to this method, the mirror surface size is small so that the deflector can be made compact. Further, this method is advantageous in that the oscillating mirror oscillates back and forth due to resonance so that the deflector can operate at high speed with low noise and low power consumption.
Moreover, the deflector operates at low oscillation frequencies and hardly generates any heat. Therefore, the optical scanning device can be housed in a housing made of a low-cost, thin, resin material containing a low percentage of glass fiber, without degrading image quality.
Patent Documents 3, 4 disclose examples in which an oscillating mirror is arranged instead of a polygon mirror.
Patent Documents 5, 6 disclose examples in which a deflector is covered by a partition member.
Patent Document 7 discloses an example in which an oscillating mirror is housed in a decompressed package.
FIG. 18 is a conceptual diagram of a simple, plate-type oscillating mirror. The following equations are satisfied, where the oscillating mirror has a length d in a direction parallel to the rotational axis, a width 2r in a direction orthogonal to the rotational axis, and a thickness t; the torsion beam has a length h and a width a; and the density of Si is ρ and the material constant is G.moment of inertia I=(4ρrdt/3)·r2spring rate K=(G/2h)·{at(a2+t2)/12}Thus, the resonant frequency f0 is obtained as follows:f0=(½π)·√(K/I)=(½π)·√{Gat(a2+t2)/24LI}The length L of the torsion beam and an oscillation angle θ are substantially proportional. Thus, the oscillation angle θ is obtained as follows:θ=κ/I·f02 (where κ is a constant)  (1)The resonant frequency f0 changes according to the spring rate K of the torsion beam, thus changing the oscillation angle θ.
Assuming that air density is η when a peripheral speed of the oscillating mirror is υ and an area is E (=2rd),
viscous resistance of air P=C·ηυ^2·E^3 (where C is a constant).
This acts as a resistance to rotation of the oscillating mirror.
A relationship between an oscillating torque T and the oscillation angle θ is expressed as follows:θ=κ′·T/K (where κ′ is a constant)  (2)
Therefore, the oscillation angle θ can be stably maintained by adjusting the current applied to generate an oscillating torque T that compensates for changes in the spring rate K of the torsion beam and air resistance.
As described above, there are problems in that the oscillation angle changes due to changes in the resonant frequency accompanying variations in the spring rate K of the torsion beam caused by temperature fluctuations or due to changes in the viscous resistance of air caused by atmospheric pressure changes.
To solve these problems, Patent Document 8 discloses a technology for detecting an oscillation angle by detecting a scanned beam, and adjusting current applied to the oscillating mirror, thereby stably maintaining the oscillation angle.
Patent Document 1: Japanese Patent No. 2924200
Patent Document 2: Japanese Patent No. 3011144
Patent Document 3: Japanese Patent No. 3445691
Patent Document 4: Japanese Patent No. 3543473
Patent Document 5: Japanese Laid-Open Patent Application No. H3-9318
Patent Document 6: Japanese Patent No. 3469387
Patent Document 7: Japanese Laid-Open Patent Application No. 2004-226651
Patent Document 8: Japanese Laid-Open Patent Application No. 2004-279947
By using an oscillating mirror instead of a polygon mirror, noise and energy consumption can be reduced, thereby providing an image forming apparatus appropriate for an office environment. Further, the housing can be made of a thin material, thereby reducing weight and costs.
However, because the oscillating mirror operates by using a resonance phenomenon, a slight change in the surrounding temperature or convections in the air causes the oscillation angle (amplitude) to change. This changes the magnification rate or write start positions in a main scanning direction, which causes color displacement or color changes, thereby significantly degrading image quality.
Even if heat is prevented from being generated by the optical scanning device itself due to employing an oscillating mirror, temperatures vary at different areas inside the housing due to heat from a fixing device or a driving device included in the image forming apparatus. Thus, the atmosphere inside the housing is not uniform, which causes convection currents in the air.
The oscillating mirror stirs the surrounding air while oscillating, and therefore, power required for rotation changes according to the conditions of the atmosphere. Accordingly, the oscillation angle (amplitude) changes periodically, which changes the magnification rate or write start positions in the main scanning direction.
In a technique disclosed in Patent Document 8, light detecting sensors are arranged outside a main scanning area at a write start position and a write end position for detecting the scanning time. Based on the detected results, rotational force applied to the oscillating mirror is specified so that the magnification rate of the total width is stabilized for a certain amount of time. However, the atmosphere in which the oscillating mirror is located changes after printing a certain number of sheets, and therefore, the rotational force needs to be frequently controlled.